Upgrades on gauges questions

I'm working on a 66 Barracuda fixing some cracks on my dash pad and replacing the factory gauges with aftermarket ones. My questions are I am replacing the amp guage with a volt guage can the same wires be used that were already being used. And also on the fuel guage by replacing it do you need to pull sending unit out to check what ohms it is or do you replace it with a different one that's matched to the guage. Also by changing out all gauges I assume that the printed circuit boards are no longer needed except for lighting on speedometer and economy guage which I can fix by adding into the lights from the other guages. Thanks just wanting to be sure